The portal vein is the main blood vessel that brings blood to the liver from the gastrointestinal tract, spleen, pancreas, and gallbladder. Right before reaching the liver, the portal vein splits into the left and right portal veins. An enlarged portal vein may occur as a result of portal hypertension (high blood pressure of the liver blood system), which is most often caused by cirrhosis (scarring of the liver) or blockages in the portal vein or veins that carry blood from the liver to the heart.